For fluid chilling packages, the most frequently used systems include two main types, namely, flooded type and direct expansion type. The former is comparatively more complicated and expensive; therefore, unless there is a special requirement for high cooling efficiency, it is often not adopted. The direct expansion type system is less complicated, but its cooling efficiency is consequently lower; therefore, finding out the cause of cooling efficiency in direct expansion type systems and providing a reasonable solution to the problem may offer a refrigeration system which is simple but has high cooling efficiency, and which is suited for use in fluid chilling packages or the like.
FIG. 1 shows a conventional and typical direct expansion type refrigeration system used in, for instance, a water chilling package. The system comprises a thermostatic expansion valve 5; an evaporator 1 connected to the expansion valve 5; a compressor 3 connected to a refrigerant outlet 13 of the evaporator 1; and a heat exchanger 4 which is commonly known as condenser connected to the compressor 3 and the expansion valve 5, wherein the expansion valve 5 has a temperature sensor 15 (such as a bulb) disposed in the vicinity of the refrigerant outlet 13 of the evaporator 1 to detect the superheat at the refrigerant outlet 13 so as to control the flow through the expansion valve 5. The expansion valve 5 conveys the liquid refrigerant in a refrigerant conduit 47 into the evaporator 1; the liquid refrigerant absorbs the heat of the cold water (when used in a water chilling package) during the process of evaporation so that the temperature of the water drops; therefore, the temperature of water passing through the evaporator 1 may be lowered. Relatively, liquid refrigerant passing the refrigerant conduit 12 absorbs the heat of the water so that when it reaches the refrigerant outlet 13 of the evaporator 1, liquid refrigerant is completely evaporated and superheated. The function of the temperature sensor 15 is, as mentioned before, to detect the superheat at the refrigerant outlet 13 of the evaporator 1, thereby regulating the mass flow rate of refrigerant passing through the expansion valve 5. Such a configuration enables the refrigerant at the outlet 13 of the evaporator 1 to be completely evaporated and superheated; otherwise, even if very little liquid refrigerant returns to the suction line 31 of the compressor 3, it may damage the compressor 3.
As is well known, the heat transfer coefficient of vapor is lower than that of two-phase mixture of liquid and vapor; in other words, the heat transfer coefficient of vaporized refrigerant is lower than that of saturated refrigerant. If the refrigerant is prevented to become completely vaporized and superheated in the evaporator, the heat exchange efficiency is undoubtedly increased. This problem has long existed in conventional direct expansion type refrigeration systems and is difficult to overcome.